1. Field of the Invention
The present invention relates to a semiconductor device having a circuit composed of thin film transistors (hereinafter referred to as TFTs) and a manufacturing method thereof. The present invention relates to an electro-optical device which is represented by, for example, a liquid crystal display panel and an electronic device on which such an electro-optical device is mounted as a part.
Note that a semiconductor device in this specification indicates a general device functioning by utilizing semiconductor characteristics, which includes an electro-optical device, a light emitting device, a semiconductor circuit, and an electronic device.
2. Description of the Related Art
In recent years, a technique of constructing a thin film transistor (TFT) using a semiconductor thin film (about several to several hundred nm in thickness) formed on a substrate having an insulating surface has drawn attention. The thin film transistor is widely applied to an electronic device such as an IC or an electro-optical device. In particular, the development for the thin film transistor as a switching element of an image display device is urgently necessary.
Various applications utilizing such an image display device are expected, and particularly the utilization to a mobile device is noted. Currently, a glass substrate or a quartz substrate is used for forming the TFT in many cases. However, there is a defect that they are easy to crack and heavy. In addition, in the case of mass production, it is difficult to use a large scale glass substrate or quartz substrate and these substrates are not suitable. Thus, it is attempted to form a TFT element on a flexible substrate, typically, a flexible plastic film.
However, the plastic film has a low heat resistance, so that it is necessary to reduce a maximum temperature of a process. As a result, under the current circumstances, a TFT having a preferable electrical characteristic cannot be formed on the plastic film as compared with the case where the glass substrate is used. Therefore, a high performance liquid crystal display device and light emitting element with the plastic film are not realized.
Also, a peeling method of peeling a layer to be peeled which is located over a substrate through a separate layer, from the substrate has already been proposed. According to the technique described in, for example, JP 10-125929 A or JP 10-125931 A, the separate layer made of amorphous silicon (or polysilicon) is provided on the substrate, and laser light is irradiated thereto through the substrate to release hydrogen contained in the amorphous silicon. As a result, gaps are produced in the separate layer, thereby separating the layer to be peeled from the substrate. In addition, according to JP 10-125930 A, it is described that a layer to be peeled (which is called a layer to be transferred in this document) is bonded to a plastic film to complete a liquid crystal display device using the above technique.
However, according to the above method, it is essential to use substrate having high transparent property. In addition, it is necessary to perform a laser light irradiation with relatively high energy enough to transmit laser light through the substrate and to release hydrogen contained in the amorphous silicon. Thus, there is a problem in that the layer to be peeled is damaged. Further, according to the above method, in the case where an element is manufactured on the separate layer, when, for example, heat treatment with a high temperature is performed in an element manufacturing process, hydrogen contained in the separate layer is diffused to reduce the concentration thereof. Thus, even when the laser light is irradiated to the separate layer, there is a possibility that peeling is not sufficiently performed. Therefore, when the amount of hydrogen contained in the separate layer is kept, there is a problem in that a process performed after the formation of the separate layer is limited. Furthermore, in the above document, it is described that a light shielding layer or a reflective layer is provided to prevent the damage to the layer to be peeled. However, in this case, it is difficult to manufacture a transmission liquid crystal display device. In addition, in the above method, it is difficult to peel the layer to be peeled having a large area.
Also, according to a conventional peeling method, a thin film is used as a layer for producing a peeling phenomenon (separate layer or the like). Thus, when nonuniformity of a film thickness is caused in the substrate, contact property between the separate layer and the substrate becomes nonuniform and poor peeling such as insufficient peeling or cracking in a substrate is easily caused at the time of peeling.